In the field of fastener elements which are attached during the manufacture of sheet metal parts to these by machines and distinguishes between pressing elements, on the one hand, and rivet elements, on the other hand. Pressing elements are characterized in that they are at least not intentionally be deformed on attachment to a sheet metal part but rather the sheet metal part itself is deformed and brought into engagement with features of shape of the pressing element, whereby the pressing element is attached in a manner secure against rotation and press-out at the sheet metal part. For rivet elements the element is intentionally deformed during attachment of the sheet metal part, mainly in order to form a rivet bead, whereby the sheet metal part is trapped between the rivet bead and a flange part in order to achieve a connection here which is also secured against rotation and press-out.
Furthermore, both pressing elements and also rivet elements are known as self-piercing elements. The designation self-piercing is to be understood such that the corresponding element pierces its own hole in the sheet metal part, naturally only when an adequate force is exerted on the self-piercing element, for example by a press or by a robot or by force-actuated tongs which press the self-piercing elements against the sheet metal part and which support the sheet metal part at the side remote from the element on a corresponding die button.
In vehicle construction it was hitherto customary to use self-piercing elements with sheet metal parts which have a strength below about 300 MPa. The self-piercing elements which are then used usually have a in addition strength in the range between 700 and 900 MPa and in exceptional cases of up to about 1250 MPa, which is entirely sufficient in order to pierce a hole in the sheet metal part on attaching the fastener element to the sheet metal part, in particular when the sheet metal part has a thickness of below about 3 mm. Strengths of the elements of up to about 850 MPa apply for example for elements of the class 8 whereas higher strengths apply for elements of the classes 10 and 12 which normally make a heat treatment and/or a specific material selection of the corresponding elements necessary.
With the fastener elements that are used, the material of the fastener element in the raw state normally has a strength of about 380 MPa. This strength is however increased to values in the range between 700 and 900 MPa solely by the cold deformation which is used in order to produce fastener element by cold deformation starting from the bar material.
For some purposes fastener elements are required which have a higher strength than 900 MPa. These are then manufactured from a material which can be hardened by a heat treatment, whereby a higher strength is achieved. However, such heat treatments are undesired in many cases. They represent a different process in comparison to manufacture of fastener elements with cold deformation and are normally not even carried out in the same factory in which the fastener elements are manufactured, whereby a considerable expenditure of time and money is required in order to produce heat treated elements of higher strength.
On the other hand, sheet metals with higher strength are increasingly been used in vehicle construction and partly also heat-treated sheet metal parts which have strength above the normal range of strength of 700 to 900 MPa for fastener elements. Such sheet metal parts of higher strength make it possible, on the one hand, to operate with thinner sheet metals, whereby weight can be saved, but, on the other hand, makes it extremely difficult to operate with self-piercing elements.
Sheet metal parts of higher strength are also required in vehicle construction which have larger thicknesses, since in some positions in the vehicle applications arise in which a thicker sheet metal part or a higher strength sheet metal part can be used to advantage.
Furthermore, interest exists currently in vehicle construction of operating at certain positions with sheet metal parts having a thickness larger than 3.5 mm, with the sheet metal parts then having strengths which lie in the above-named region of 300 MPa. An example for such applications is in the chassis area of trucks.